1. Field of the Invention
The present invention relates to a digital image processing technique and more particularly, to an apparatus for compressing image used in coding image data. The invention also related to an image coding and synthesizing method by use of such an image compressing apparatus.
2. Description of the Related Art
In recent years, image display devices making use of electronic means, such as large-size projectors, are becoming popular as means for synthetically displaying a plurality of image data in conferences, lecture meetings and so forth, in place of conventional display devices such as OHPs (overhead projectors), slide projectors and so forth. Such electronic display devices are finding spreading use by virtue of reduction in the costs of semiconductor devices such as memories. Usually, this type of display device employs a personal computer which forms from a plurality of image data a synthetic image which is to be displayed on a large-size screen.
The operation for forming a synthetic image from a plurality of image data by a personal computer or the like is conducted by picking-up a plurality of images such as color photos, output of a wordprocessor or the like by means of a camera, an image scanner or the like, and the picked-up images are digitized and stored in memories. Then, an editorial operation is conducted such as enlargement, contraction, cut-out, layout and so forth so that a synthetic image to be displayed is formed. The thus formed synthetic image is then recorded in a suitable recording medium such as a magnetic recording disk or a floppy disk, and is conveyed to the place where a conference or lecture meeting is to be held for presentation of the synthetic data through display.
A description will be given of the conventional method for synthesizing an image. It is assumed here that rectangular pictorial parts P.sub.1, p.sub.2 and p.sub.3 shown in FIG. 1b are superposed to a total background image B shown in FIG. 1a so as to form a synthetic image A as shown in FIG. 1C. The following methods are available for coding, storing and reproducing this synthetic image A.
Method I: A method shown in FIG. 2a in which the whole synthetic image A is regarded as a single image, compressed and coded for recording. For reproducing, the coded data are decoded to enable display of image. PA0 Method II: A method shown in FIG. 2b in which the total background image B and the pictorial parts P.sub.1, P.sub.2 and P.sub.3 are independently compressed and coded for recording. For reproducing the images, the coded data are decoded and portions of the data of the total background image B corresponding to the positions of the component images P.sub.1, P.sub.2 and P.sub.3 are replaced with the decoded data of pictorial parts P.sub.1, P.sub.2 and P.sub.3 so that a synthetic image is formed for display.
Since such a system for synthesizing an image from a plurality of image data deals with image data itself, there rises a problem that a large amount of data should be processed. The large quantity of image data poses a problem particularly when the synthesized image is to be presented to a conference or lecture meeting. Namely, in such a case, it is preferred to use a compact recording medium and, therefore, to reduce the quantity of data. There is proposed therefore a method to reduce the quantity of data in compressing a full-color image by use of the fact that the human eyes are rather insensitive to high spatial frequency domain.
This method, however, suffers from the following problems. Namely, in some cases, the synthesis is conducted in such a manner that some pictorial parts 62 is superposed to a full-color total background image 61 and, further, a character 63 is superposed for explanation of the image, as shown in FIG. 3. If such a full-color image on which the pictorial parts and the character are superposed is compressed in the above mentioned method making use of the insensitivity of human eyes in the high spatial frequency domain, the qualities of the images of the pictorial parts and the text image are degraded when the compression is conducted at a compression ratio ordinarily used in the compression of full-color image. Conversely, a compression ratio which can eliminate such a degradation of the pictorial parts and text images is too low to achieve a convenience of compression.
It is also to be noted that presentation in a conference or a lecture meeting sometimes can be made more effective when a stress is put on a portion of the display image by changing the display of such a portion.
In the method I mentioned before, it is necessary to preserve the whole A of the synthetic image. It is therefore impossible to make common use of the data of the total background image B, as well as data of the independent pictorial parts P.sub.1, P.sub.2, P.sub.3. Accordingly, a large storage capacity is required because the storage has to be conducted on the basis of the synthesized image A which is composed of a large quantity of data. It is also impossible to move the component images P.sub.1, P.sub.2, P.sub.3 separately and independently on the total background image.
Furthermore, the methods I and II mentioned before suffer from the following common drawbacks. It is assumed that a pictorial parts P.sub.2 having an object image O.sub.2 of a chair and a component background image b.sub.2 is superposed on a pictorial parts p.sub.1 having an object image O.sub.1 of a tree and a component background image b.sub.1. There is no means for discrimination between the object images O.sub.1, O.sub.2 and the component background images b.sub.1, b.sub.2. Therefore, the synthesized image may have a drawback in that, as shown in FIG. 4, the portion of the object image O.sub.1 is hidden behind the corresponding portion of the component background image b.sub.2 so as to become invisible. Furthermore, when the object image O.sub.2 of a chair has regions in closed loops such as H.sub.1 and H.sub.2, the component background image b.sub.2 of the pictorial parts P.sub.2 appear in these regions H.sub.1, H.sub.2, although the total background image B is expected to appear through these regions H.sub.1, H.sub.2.
Referring now to FIG. 2b, in case that the pictorial parts P.sub.1, P.sub.2, P.sub.3 are compressed and coded according to the method II mentioned before, since the pictorial parts P.sub.1, P.sub.2, P.sub.3 includes the component background images b.sub.1, b.sub.2, b.sub.3 in addition to the object images O.sub.1, O.sub.2, O.sub.3 of a tree, a frame and a chair respectively, the compression of these pictorial parts P.sub.1, P.sub.2, P.sub.3 inevitably includes a large amount of useless information, resulting in a long computing time.